CA1107750A - Propoxylated ethoxylated surfactants and method of recovering oil therewith - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A method for recovering oil from a subterranean formation is disclosed wherein a novel solution is injected into and driven through the formation. The solution contains an effective amount of surface-active agent having the general for formula R1O(C3H4O)m (C2H4O)nYX

wherein R1 is a linear or branched alkyl radical, an alkenyl radical, or an alkyl or alkenyl substituted benzene radical, the non-aromatic tic portion of the radical contain-ing from about 6 to about 24 carbon atoms;
m has an average value of between about 1 and about 10;
n has an average value of between about 1 and about 10;
Y is a hydrophilic group; and X is a cation, preferably monovalent.

Description

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2 1. ~ield of the Invention

3 This invention relates to a method for recovering oil from a

4 subterranean formation, and more particularly to special surfactant systems to be used with waterflooding techniques to improve the oil displacement 6 efficiency of waterfloods.
7 2. Description of the Prior Art 8 ~he petroleum industry has recognized for many years that only a 9 portion of the original oil in an oil reservoir can be produced by what is referred to as "primary recovery," i.e. where only initial formation energy 11 is used to recover the crude oil. It is also well known that conventional 12 methods of supplementing primary recovery are relatively inefficient.
13 Typically, a reservoir retains over half its original oil even after the 14 application of currently available "secondary" recovery techniques. Accord-ingly, there is a continuing need for improved recovery methods which will 16 substantially increase the ultimate yield of petroleum from subterranean 17 reservoirs.
18 "Waterflooding" is by far the most economical and widely practiced 19 of secondary recovery methods. In such a process, water or other aqueous fluid is introduced through injection wells to drive oil through the forma-21 tion to offset producing wells. Much of the current work in secondary 22 recovery technology has been directed toward improving the efficiency of 23 waterflooding processes.
24 Surface-active agents or surfactants are one class ~f materials which have been proposed for improving the efficiency of waterflooding 26 processes. Much of the oil that is retained in the reservoir after a 27 typical waterflood is in the form of discontinuous globules or discrete 28 droplets which are trapped within the pore spaces of the reservoir.

7~ 3 1 Because the normal interfacial tension between the reservoir oil and water 2 used for flooding is so high, these discrete droplets are unable to deform 3 sufficiently to pass through narrow constrictions in the pore channels of 4 the formation. When surfactants are added to the flood water, they lower the interfacial tension between the water and the reservoir oil and permit 6 oil droplets to deform, coalesce and flow with the flood water toward pro-7 ducing wells. It is generally accepted that the interfacial tension between8 the surfactant-containing phase and the reservoir oil must be reduced to 9 less than O.l dyne/cm for low-tension flooding to give effective recovery.
One difficulty in the use of surfactants in general and anionic 11 surfactants in particular is their tendency to be depleted from the injected 12 solution as the solution moves through the reservoir. The surfactants tend13 to be lost by precipitation as insoluble salts of ionic materials, such as 14 polyvalent metal ions, that may be dissolved in the fluid in the reservoir;by adsorption on the reservoir rocks; by physical entrapment within the 16 pore spaces of the rock matrix; and/or by chemical conversion, such as 17 hydrolysis of an active component of the surfactant system to a component 18 that is insoluble, inactive, or detrimental in that system. If the surface-19 active agent is removed from the waterflood solution as it moves through the reservoir, the agent is not available to act at the oil/water interface.
21 Quite naturally, surfactant depletion decreases oil recovery efficiency.
22 In a waterflood oil recovery process where the water contains a 23 surfactant, the efficiency of the oil displacement is strongly affected by 24 (l) the rate of surfactant loss and (2) the surface activity (extent of lowering of the oil/water interfacial tension) of the surfactant.
26 Another difficulty observed in the use of many anionic surfactants 27 is the inability of the surfactant to exhibit high surface activity in high28 temperature reservoirs (i.e., temperatures of about l20D F or more) and/or 7~`'3 1 in aqueous solutions containing high concentrations of inorganic salts.
2 Thus, many oilfield brines, e.g. formation brines which are considered 3 herein to contain high concentrations of inorganic salts, generally contain 4 over 2% NaCl and over 0.5% CaCl2 and MgCl2 total; concentrations of over 4%
NaCl and 2% CaCl2 and MgC12 are common. (All percentages reported herein 6 are percents by weight, unless otherwise noted.) As used herein, the term 7 formation brine includes not only brine originally present in the formation 8 but also brines subsequently introduced into the formation, e.g. during 9 flooding operations. Of course, formation brines are encountered which do not contain such high concentrations of inorganic salts, but generally such 11 brines are less common and in any event still contain what would be consid-12 ered "high" concentrations of inorganic salts by most standards.
13 When the typical salt concentrations of many formation brines are14 considered, it is not surprising that development of suitable surfactants for reservoir environments has met with little success. As mentioned, 1~ formation brines often contain concentrations of sodium chloride ranging 17 from 2% to over 10%, and combined calcium and magnesium chloride concentra-18 tions from 0.5% to over 2.0%. These concentrations may range up to the 19 solubility limits of said salts in water at formation temperature. To place the development of surfactants useful under reservoir conditions in 21 perspective, surfactants useful as detergents in hard water contemplate 22 salt concentrations which are orders of magnitude less, e.g. sodium chloride 23 concentrations no higher than about 0.2% and combined calcium and magnesium24 chloride concentrations of no more than 0.05. In f~ct, most detexgent surfactants are entirely unsuited for use in reservoir environments.
26 Further examples of the typically high formation brine salt 27 concentrations may be found in A. G. Ostroff, "Introduction to Oilfield 28 Water Technology", p. 5 (1~65). For comparision, typical water hardness `7~

1 values in the detergent art may be found in K. Durham, "Surface Activity 2 and Detergency," p. 96, 98 and 137-142 (1961).
3 The problem is further complicated by the fact that although a 4 given surfactant may be soluble in formation brines, this is no assurance that it will be effective in lowering interfacial tensions sufficiently for 6 enhanced oil recovery.
7 Generally, as the temperature of the reservoir and concentration 8 of inorganic salts in the brine solution of the reservoir increase, the 9 surface activity of ccnventional anionic surfactants decreases. Surfactantshave been suggested which exhibit some tolerance to either high temperature 11 or high salt concentrations. None of these surfactants, however, have the 12 ability to exhibit a high degree of surface activity under all types of 13 reservoir conditions, including high salt concentrations or high temperature, 14 or both high temperature and high salt concentration reservoirs.
It has generally been found that positioning an ethoxy group 16 adjacent to the sulfonate or sulfate group of a given surfactant tends to 17 increase the solubility of such surfactant in water; moreover, increasing 18 the number of ethoxy groups tends to increase the water solubility of such 19 surfactants and also provides improved solubility characteristics in water having high concentrations of inorganic salts such as sodium chloride, 21 magnesium chloride, and calcium chloride. Accordingly such surfactants 22 have been proposed for use in environments having high concentrations of 23 such inorganic salts.
24 Thus, it has been reported in U.S. Patent ~o. 3,508,612, issued to Reisberg et al, that a two-component surfactant mixture exemplified by 26 a petroleum sulfonate and a salt of a sulfated polyalkoxylated alcohol 27 (e.g. C12 150~C2H40)3SO3Na) exhibits improved tol~rance against high salt 28 concentration environments. It has been four,d, however, that the surface ~L~ ~ 7 ~

1 activity of the two ingredients in this composition is very sensitive to 2 the salt content of the brine. This sensitivity is important because in 3 commercial operations concentrations may vary from time to time and because 4 the concentration of the surfactant composition will vary as it moves through the formation because of mixing with in-situ water, non-uniform 6 movement and the like. This results in both surfactant loss to the formation 7 and loss of surface activity.
~ A major cause of the problems associated with the use of polyalkoxy-9 lated surfactants is due to the fact that as the fluid containing these surfactants moves through a formation, a chromatographic-type separation of 11 surfactant components takes place. This may be better understood by 12 considering the general chemical composition of a polyalkoxylated surfactant.
13 Generally, such surfactants are prepared by alkoxylating a suitable organic 14 substrate using a strong base or a Lewis acid catalyst such as sodium hydroxide, BF3 or the like, followed by sulfation or sulfonation. While 16 the resulting product may be purified to some extent, the resulting "pure"
17 surfactant ultimately used in the field is, in reality, a mixture of discrete 18 compounds, each having been alkoxylated to a different extent. ~or example, 19 when ethoxylating a primary alcohol, ROH, the resulting product may be represented as RO (E)4 2H (where EO represents the ethoxy group (C2H40) 21 and the subscript 4.2 indicates the average number of ethoxy groups).
22 Actually, the product comprises a mixture of adducts and unreacted alcohols, 23 for example 24 R OH, RO(EO)lH, RO(EO)2H, RO(EO)3H, RO(EO)4H, RO(EO)5H, ~O(EO)6H,..., RO~EO)l2H, --26 and so forth, in different proportions such that an average value of 4.2 27 ethoxy groups has been achieved. It has been found that each component 28 compound demonstrates a substantially different surface activity at a given 29 concentration of inorganic salts.

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1 Thus, while the surfactant may possess some properties similar to 2 a purified, single component surfactant and may give acceptable results in 3 small scale laboratory tests, field results can vary substantially due to 4 extensive chromatographic-type separation of one or more components during flooding. A less subtle variation of this problem has been noted previously 6 where a mixture of two (or more) different types of surfactants (co-7 surfactants) are utilized, such as the approach suggested in U.S. Patent 8 No. 3,811,505, issued to Fluornoy et al.
9 Thus, as the surfactant-containing fluid moves through the vast distances of a reservoir (compared to short laboratory cores), the various 11 surfactant components 6eparate. Each separated component is in a similar 12 environment, i.e. reservoir brine of a given inorganic salt concentration.
13 However, except for that discrete component whose properties are acceptable 14 in the reservoir brine, all remaining components exist in an unacceptable environment. In other words, all other discrete components, when separated, 16 will demonstrate poor surface active properties in the reservoir brine.

17 SUMMARY 0~ THE INVENTION
18 The present invention relates to a flooding process and a composi-19 tion used in such flooding process, which alleviates the above problems.
In accordance with this invention, a process is provided for recov~ring oil 21 from a subterranean formation wherein fluid containing a surfactant is 22 injected into the formation. The surfactant provides a high degree of 23 surface activity in reservoirs having a high concentration of inorganic 24 salts. The surfactant is selected from the group of compounds characterized by the general formula:

7 ~ ~ ~

RlO(c3H60)m (C2H4)n 2 wherein 3 Rl is a linear or branched alkyl radical, an alkenyl 4 radical, or an alkyl or alkenyl substituted benzene radical, the non-aromatic portion of the radical contain-6 ing from 6 to 24 carbon atoms;
7 m has an aver~ge value of from about 1 to about lO;
8 n has an average value of from about l to about 10;
9 Y is a hydrophilic group; and X is a cation, preferably monovalent.
11 The order of the alkoxy groups has been discovered to be highly 12 important in alleviating the problems outlined above.
13 In a preferred form of the invention, Y is a sulfate or sulfonate 14 group and Rl is a branched alkyl radical having at least two branching groups.
16 Where Y is a sulfonate group, a general formula for Y may be 17 written as:

wherein 21 R2 is an alkyl, cycloalkyl, alkenyl, alkylaryl or aryl 22 radical containing up to 8 carbon atoms;
23 R3 is a hydrogren atom, a hydroxyl radical, or an aliphatic 24 radical containing from ]-~ carbon atoms;
Most preferably, m and n each range be~ween about 2 and about 6.
26 Ranges of optim~m average.numbers of propoxy and ethoxy groups ~m and n) 27 for different salt concentrations have been discovered and this further 28 enhances the advantages o these surfactants. The surfactant system of the 29 invention is~ therefore9 highly effective in br-ines commonly found in oil ield envirol~ments.

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1 The agents of this invention can be used in various fluids 2 including an aqueous solution, an oil solution, a microemulsion which is 3 miscible with the crude oil and/or formation water, or a microemulsion 4 which is immiscible with the formation crude oil and/or formation water.
The agents as provided in this invention have good resistance to 6 precipitatisn and adsorption when passing through a formation and will 7 effectively reduce interfacial tension between the injected fluid and the 8 in-place crude oil even in brine environments wherein high concentrations 9 of inorganic salts exist. Moreover, the sulfonates in particular exhibit very good resistance to hydrolysis in high-temperature reservoirs.
11 Most importantly, many discrete component compounds of the surfac-12 tant, resulting from the varying degrees of alkoxylation, exhibit similar 13 surface active properties for a given concentration of inorganic salts.
14 Therefore, the effect of chromatographic separation of some of the surfactant components in a formation during flooding is significantly reduced, thereby 16 enhancing oil recovery. This is believed to be due to the order in which 17 alkoxylation is accomplished, i.e. propoxylation first and ethoxylation 18 second. This is apparently due to modification of the water-ethoxy group l9 interaction provided by the presence of the propoxy groups. These agents ~herefore offer significant advantages over the agents used heretofore.

21 RIEF DES~RIPTION _F THE ~RAWING
22 The drawing is a plot illustrating optimal combinations of 23 propoxy and ethoxy groups in a surfactant of the invention for given salt 24 concentrations at 74F and in admixture with diesel fuel.

~ :~ ~ 7 7 ~ i~

2 The benefits and advantages which can be obtained in the practice 3 of this invention are achieved through the use of a new and improved class 4 of surface-active agents. As will be discussed in more detail hereinafter, these agents can be used in any type of surfactant flooding process for 6 recovering crude oil from a subterranean oil-bearing formation. They are 7 particularly useful in reservoirs having high concentrations of inorganic 8 salts.
9 In the practice of this invention, a solution is provided which contains an effective amount of a surface-active agent selected from the 11 group of compounds having the general formula:
12 R10 (C3H60)m (C2H40)n YX

13 wherein 14 Rl is a linear or branched alkyl radical, an alkenyl radical 7 or an alkyl or alkenyl substituted benzene 16 radical, the non-aromatic portion of the radical contain-17 ing from 6 to 24 carbon atoms;
~8 m has an average value of from 1 to 10;
19 n has an average value of from 1 to 10;
Y is a hydrophilic group; and 21 X is a cation, preferably monova]ent.
22 Compounds which comply with the above formu]as will be referred 23 to herein as "compound (a)".
24 Y is a suitable hydrophilic group or substituted hydrophlilic ~5 group such as, for examp1e, the sulfate, sulfonate, phosphate or carboxylate 26 radical. Preferably, Rl is a branched alkyl radical having at least two 27 bra~ching groups and Y is a sulfate or su1fonate group ~here Y is a 28 sulfonate group, the preferred structure of Y may be characterized by the 29 following general formula:

L~. `7 ~iC~

3 wherein 4 R2 is an alkyl, cycloalkyl, alkenyl, alkylaryl or aryl radical containing up to ~ carbon atoms; and 6 R3 is a hydrogen atom, a hydroxyl radical, or an aliphatic 7 radical containing from 1-8 carbon atoms.
8 It should be understood that the surfactant of the present inven-9 tion is not a pure substance in the strict sense, but really a mixture of components distributed such that m and n are the resulting average values.
11 Surfactants of the general formula of compound (a) can be prepared 12 in a number of ways. For the sake of brevity and clarity, however, only 13 the preferred method of preparation will be presented herein.
14 Suitable precursors of compound (a) include a C6 24 linear or branched alcohol, a C6_24 methyl phenol, or a C6_24 dimethyl phenol-16 Preferably, the precursor is a branched-chain alcohol having at least two 17 branching groups. More preferably~ the branched chain alcshol contains 18 from 10 to 16 carbon atoms. The symbol C6 24 is used herein to designate 19 an alkyl radical having from 6 to 24 carbon atoms per ~olecule.
Any isomers or substituted d~rivatives of these precursors are 21 suitable in the practice of this invention. However, the use of a branched 22 chain alcohol having from 10 to 16 carbon atoms is especially pre~erred and 23 results in improved salinity and multivalent eation tolerance.
24 The alcohol or substituted derivatives is reacted ~ith propylene oxide (P~) to yield an alkyl oxypropylene ether or an alkyl phenyl oxypropy-26 lene ether. The average number of propylene oxide units (i.e., the value 27 of ~) can be varied as desired up to about 10, except that at least one ~ ~ 7 . ~ ~

1 propoxy unit must be added. Preferably, m ranges from a value of about 2 2 to a value of about 6.
3 The resulting oxypropylene ether is then reacted with ethylene 4 oxide (EO) to yield an alkyl oxypropylene oxyethylene ether or an alkyl phenyl oxypropylene oxyethylene ether. The average number of ethylene 6 oxide units added (i.e., the value of n) can be varied between about 1 and 7 about 10. Preferably, however, n ranges between about 2 and about 6.
8 Methods of alkoxylation are well known to those skilled in the 9 art. The alkoxylation reaction can be achieved using a strong base or Lewis acid catalyst such as NaOH, KOH, BF3 or SnC12. Examples of other 11 suitable bases include sodium phenolate and al~ali metal alkoxides such as 12 sodium methoxide or propoxide. Other suitable acids include BF3-etherate, 13 p-toluene sulfonic acid, fluorosulfonic acid, aluminum butyrate and per-14 chloric acid. The following example will serve to illustrate an alkoxylation procedure.

16 Example 1 17 A total of 453 g (2.27 moles) of i-tridecyl alcohol and 6 g of 1~ dry sodium hydroxide was charged to a 2-liter stirred autoclave reactor.
19 The reactor was purged with nitrogen and heated to appro~imately 140C.
Four hundred grams of propylene oxide (6.90 moles) were delivered to the 21 reactor over a period of about an hour. The reaction temperature was 22 increased to about 160C until reactor pressure dropped to a constant 23 value. The reaction mixture was then cooled to about 140C, after which 24 399 g of ethylene oxide (9.07 moles) were delivered to the reactor over about a one-half hour period. The reaction temperature was again raised to 26 about 160C and maintained until the pressure dropped to a constant value.

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1 After cooling, approximately 1200 g of product were withdrawn and filtered 2 through a sand bed to remove ~odium hydroxide particles. Dissolved water 3 was removed by distillation. Analysis of the product indicated that an 4 average of three propoxy groups and four ethoxy groups were added to the i-tridecyl alcohol precursor.

6 To the resulting propoxylated/ethoxylated material can then be 7 added a suitable hydrophilic group (Y). The choice of hydrophilic group 8 depends on availability, expense, the degree of surface activity, and 9 hydrolytic stability imparted to the resulting surfactant. Using these criteria, suitable groups include the sulfate group, the sulfonate group, 11 the phosphate group and the carboxylate group. In particular, we have 12 found that the incorporation of the sulfate or sulfonate groups into surfac-13 tants of the present invention results in improved brine tolerance. Prefer-14 ably, therefore, the hydrophilic group is sulfate or sulfonate.
The following example will serve to illustrate the preferred 16 sulfation procedure.

17 Example 2 18 The product from Example 1 was added to a wiped film reactor at 19 a rate of about 12 g (0.0214 g-moles) per minute. Sulfur trioxide, diluted in a stream of dry nitrogen, was then introduced at a rate of 2.0 g ~0.0250 21 g-moles) per minute. The sulfate acid product was collected continuously 22 at the outlet of the reactor. The resulting product was then neutraliz~d 23 with fifty weight per cent sodium h~dro~ide solution to give a surfactant 2~ of compositiOn i-C13 ~27(C3~6)3 (C2H4 )4 3 While a large number of suitable alkoxylated compounds may be 2~ prepared, each must be prepared by a method which introduces propoxy 27 groups first and ethoxy groups second.

3~

1 Alternatively, the sulfation step can be omitted and the resulting 2 propoxylated ethoxylated composition may be reacted with any suitable 3 alkali metal (including, for example, sodium, potassium, or lithium) as 4 the next step to form a sulfonate surfactant. The reaction product will be referred to herein as a metal etherate for the sake of brevity.
6 Thus~ to form the sulfonate surfactant, the metal etherate can be 7 reacted with a large number of compounds to yield surfactants generally 8 characterized by compound (a). For example, the metal etherate may be 9 reacted with chloromethyl sulfonate, vinyl sulfonate, 1,3-propane sultone, ~0 or 1,4-butane sultone to prepare compounds having a general structure where 11 R3 is a hydrogen atom. The metal etherate may also be reacted with 3-12 methylpropane sultone, or 4-methylbutane sultone, to prepare compounds 13 having a structural formula where R3 is a methyl group. The metal etherate 14 also may be reacted with hydroxyvinyl sulfonate, 3-hydroxypropane sultone, or 4-hydroxybutane sultone to prepare compounds having a structure where R3 16 is a hydroxyl group.
17 The sultones used for the sulfonation of the metal etherate are 18 cyclic esters of hydroxysulfonic acids. The name sultone is derived from 19 their formal resemblance to lactones. Considerable literature has been devoted to sultones and the chemistry of the propane and butane sultones in 21 well known to the art. See, for example, R. F. Fisher, ndustrial and 22 Engineering ~hemistry, Vol. 56 No. 3, March 1964, pp. 41-45.
23 Alternatively, the propoxylated ethoxylated material can be 24 reacted with phosphorous pentoxide to form a phosphate. If desired, a catalyst such as BF3-etherate complex may be used. The resulting product 26 is then neutralized with sn alkali metal base such as sodil~m or potassium 27 hydroxide, or sodium or potassium carbonate, or the like, to form an alkali 28 metal salt.

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1 Alternatively, a carboxylate group may be incorporated into the 2 propoxylated ethoxylated material by any number of well-known methods. ~or 3 example, the propoxylated ethoxylated material may be reacted with a halogen 4 carboxylic acid to result in a carboxylic acid. Alternatively, the alcoholic end group - CH20H can simply be oxidized under well-known conditions to 6 yield a carboxylic acid. The resulting product is then neutralized using 7 an alkali metal base to form a carboxylate surfactant having the general 8 formula of compound (a).
9 The techniques of alkylation, propoxylation, ethoxylation, sulfation, sulfonation, phosphation and carboxylation which are used to 11 prepare the various surfactant embodiments of this invention are generally 12 well-known in the art. Accordingly, the preparation of the surfactants of 13 this invention will not be exemplified further since the techniques for 14 production are well-known.
The reservoir temperature, the crude oil in the reservoir and the 16 salinity of the fluid to be used for flooding operations will dictate the 17 preferred forms of compound (a). ~lthough it has been found, in general, 18 that the incorporation into the surfactant of propoxy groups and ethoxy 19 groups in the order described above results in a mixture of compounds, a great many of which have surface active properties that are much less 21 sensitive to salinity and to the presence of multivalent cations, there 22 still exist preferred combinations of prGpoxy groups and ethoxy groups for 23 given concentrations of inorganic salts. That is, for a given brine there 24 exists an optimum family of surfactants defined by a set of pairs 2s {~m, n)}, of average values m and n.
26 The drawing illustrates this feature. It was prepared from 27 experimental data using brines composed of various percentages of a formation 28 brine (speci~ically Tar Springs Brine, hereinafter "TSB") having the compo-29 sition shown in Table I.

r~ ~ 3 1 Table I - Composition of Tar Springs Brine 2 ComponentConcentration (g/l.) 3 NaCl 91.71 4 CaC12 7.86 MgC12 6H2 10.33 BaC12 2H2 0.133 7 NaHC03 0.195 8 In the drawing, the contour labelled 1.0 corresponds to 100% TSB
9 and has a brine composition identical to that shown in TABLE I. The contour labelled 1.5 has 50% more of each component listed in TABLE I than does 11 TSB, and so on.
12 Thus where the the flooding fluid has a concentration of inorganic 13 salts on the order of 110% TSB, it has been found that for an i-tridecyl 14 propoxylated ethoxylated sulfate surfactant an optimum pair is three propoxy groups and two ethoxy groups ~m=3 and n=2); another optimum pair i~ m=4 and 16 n=3; ~till another is m=6 and n=4, and so on. Conversely stated, for a 17 given family of surfactants defined by {(m, n)}, there exists an optimum 18 brine composition (e.g. optimal salinity) for best results.
19 It can also be noted that, for the surfac~anc under consideratiorl, the curves defining the optimwn ratio are not always linear. Further, 21 there is a region for simultaneously low m and n values that defines surfact-22 ants which, if used, give undesirable gels. For an i-tridecyl alcohol 23 precursor, it has been found, for instance, that the sum of m and n should 24 preferably be greater than about tWG. The delineating summation value may, 2~ however, vary depending on the nature, chain ]ength, or branching of the 26 substrate which is alkoxylated, on the temperatllre, the na~ure of the oil, 1 the type and amount o~ co-surfactant or co-solvent (if any), and the quantity2 and type of unreacted alcohol and alkoxylates.
3 It can further be noticed that for low ~alues of m and especially 4 for m=0 (implying an ethoxylated-only surfactant), optimal salinity is extremely sensitive to a change of only one mole of ethylene oxide, i.e.
6 for Qn = +1. Thus, referring to the drawing, when m = 0 and n = 3, decreas-7 ing the degree of ethox~71ation by one (~n = -1~ results in a change in 8 optimal salinity of 40~ TSB~ By comparison, when m = 3 and n = 3, increasing9 or decreasing the average degree of ethoxylation by one results in a change in optimal salinity of only +15~ TSB, respectively. Similarly, increasing 11 or decreasing the average degree of propoxylation by one results in a 12 change in optimal salinity of only about +20~ TSB, respectively. Thus, for 13 ethoxylated-only surfactants, the change in optimal salinity is at least 2 14 times greater than the change seen for the propoxylated-ethoxylated ~urfac-tants. It follows that the salt sensitivity of the coT~ponent species in 16 the ethoxylated-only surfactants is much greater than that of surfactants 17 prepared through propoxylation followed by ethoxylation.
18 Various compounds having a general formula as characterized by 19 compound (a) can be used in the practice of this invention. ~xamples of suitable surfactants may include compounds whereln Rl is the i-tridecyl 21 group, i-hexadecyl group, i-decyl group, or i-dodecyl benzene, toluene or 22 xylene group; R2 may be a methyl, ethyl, propyl, butyl, cyclopentyl, cyclo-23 hexyl, or benzene radical (in the case of a sultone); R3 can be a hydrogen 24 atom, a me~hyl, ethyl, propyl, ethanyl, propenyl, or hydroxyl radical. It is contemplated that when ~1 contains an aromatlc group, the group will be 26 a single benzene radical which will always be alkyl or alkenyl substituted.
27 X can include alkali metals such as sodium, potassium and lithium,~8 alka]ine earth metals such as calcium and barium, amines including alkanol 29 amines and their oxyalkylated adducts, and ammonium. ~ny other examples 1 of Rl, R2, R3 and X will be apparent to those skilled in the ~rt, and the 2 specific examples listed should not be construed as limiting.
3 To further illustrate how the surface-active agents disclosed 4 herein achieve the benefits of this invention it may be helpfu] to generallydiscuss the structure of a surfactant and the surfactants of this invention 6 in particular. Most conventional surfactant molecules have ~n amphiphilic 7 structure; that is, the mo]ecules are composed of groups which have opposite8 solubility tendencies. For use in oil recovery operations the molecules 9 should have at least one predominantly lipophilic or oil-soluble group and at least one predominantly hydrophilic or water-soluble group. These are 11 present in a certain ratio or balance. Where the correct balance is achieved, 12 the surfactant will be able to so]ubilize relatively large amounts of both 13 oil and water at or near optimal salinity. Such balanced surfactants ~re 14 highly surface active and have been found very effective for oil recovery.
This desired balance can be achieved much more accurately with 16 surfactants formulated in accordance with this invention than with other 17 conventional surfactants because of the presence of prop3xy groups between 18 the ethoxy groups and the lipophilic group ~Rl). W~lile not wishing to be 19 ~ound by theory, the benefit of the present invention may be e~plained as follows. Ethylene oxide (E0) chains interact very strongly with waterl so 21 that deletion or insertion of a single E0 group vastly changes the surface 22 activity of the molecule in a given brir.e enviro~nent, as noted previously.
23 However, propylene oxide chains intersct less strongly with water, thus 24 reducing the overall hydrophile-water interaction and hence, as noted previously, reducing the impact on optimal salinity of a change of one 26 ethoxy or propoxy group. In summary then, an important feature of surfac-27 tants of the present invention is that ~he presence of the propoxy groups ~8-r~ r-~

1 bet~een the lipophilic group and the ethoxy chain/hydrophilic group (Y) 2 modifies the overall hydrophile-water interaction energy, thus permitting 3 accurate design of surfactants having the desired balance. Hence, such 4 surfactants exhibit a large capacity for oil-water solubilization, high surface activity, decreased sensitivity to brine, and good oil recovery.
6 Surface-active agents in accordance with this invention can be 7 formulated which exhibit this desired oil-water solubilization for use in 8 the adverse environments of high-temperature reservoirs and/or high salt 9 concentration environments. In addition, a large number of the component compounds of the surfactant used will exhibit similar or equivalent surface 11 active properties for a given formation brine thereby substantially dimin-12 ishing the problems ordinarily associated with chromatographic separation 13 during flooding operations. While separation still occurs, as with other 14 surfactants, a large number of the components still give acceptable surface activities under reservoir conditions, significantly enhancing recovery.
16 As pointed out in the following discussion, such compounds can be formulated 17 by propoxylating and ethoxylating the lipophilic group in a specific order;
18 the properties of a surfactant may further be varied by adjusting the 19 relative size and character of the hydrophilic or lipophilic portion of the surface-active molecule.
21 Generally, the oil solubility of surfactants is related to the 22 molecular weight of the lipophilic (oil-soluble) portion of the molecule.
23 Since a surfactant's affinity for water generally increases faster than its 24 affinity for oil as temperature increases, the ability to increase a surfactant's oil solubility can be very important. The surfactants of this 26 invention, characterized by compound ~a), can be designed to take into 27 account adverse temperature effects by choosing Rl of a suitable molecular 28 weight to exhibit a desired oil solubility for a particular reservoir 29 temperature.

l Moreover, it has been discovered that the surface activity of 2 surfactants of the present invention may be further enhanced by using a 3 branched chain alkyl lipophilic portion.
4 The hydrophilic portion of the family of surface-active agents characterized by this invention can be adjusted to increase the agents' 6 water solubility. The preferred sulfate or sulfonate radical is one 7 portion of the surface-active agent which gives the molecule some hydro-8 philicity. Typically, sulfated and sulfonated surfactants tend to exhibit 9 a relatively high degree of water solubility. However, if a surfactant must rely only on the sulfate or sulfonate radical for its hydrophilicity, 11 the surfactant's solubility in brine will decrease as water salinity 12 increases. It is well known that an ethoxy group positioned between the 13 lipophilic group and the hydrophilic group will increase the surfactants' 14 solubility in water; moreover, it is also known that increasing the number of ethoxy groups will increase the surfactants' water solubility.
16 It has also been found, however, that the optimum surface-active 17 properties of an ethoxylated-only surfactant in a medium containing inorganic 18 salts are highly dependent on the number of ethoxy ~roups contained in a 19 molecule. Thus, suppose an ethoxylated-only surfactant was required that would be ef~ective in a brine containing salts equivalent to those in 190%
21 TSB. The formula for one such surfactant is C13H270(E0)~ 5 S03 Na , where 22 2.5 is the average number of ethylene oxide groups. However, this surfactant 23 is a mixture of several pure componen~ surfactants having the formulae 24 C13H270(E0) S03 Na , where n = 0, 1, 2, 3, 4, ..~ and so on. The amounts of each of these pure componPnts varies in such a way that their mole-26 average is 2.5. In test-tube experiments designed to estimate surface 27 activity in terms of capability to solubilize water and oil, it is found ~.7 ~` ~i~3 1 that this mixture is quite effective for reducing interfacial tension in 2 an environment containing salts in concentrations approximately equivalent 3 to those in 190% TSB. However, in long-core flooding experiments or in 4 field applications, the various pure-component surfactants are subject to chromatographic separation and none of the separated surfactants 6 exhibits acceptable surface activity in the resident brine. Instead, 7 for example, C13H270(E0)2S03 ~a is effective only near 170% TSB and 8 C13H270(E0)3S03 Na is effective only near 210% TSB.
9 Surprisingly, it has been found as a feature of this invention that when a certain number of propoxy groups are incorporated adjacent 11 the lipophilic portion of the molecule, followed by ethoxy groups, the 12 surface active properties of the resulting surfactant (actually a mixture 13 of compounds) are less sensitive to salinity or higher concentrations of 14 multivalent cations. That is, many of the discrete compounds that comprise the surfactant mixture demonstrate similar surface active 16 properties. This means that the inorganic salt concentrations of the 17 flooding fluid can be adjusted to match that of the form~tion; if 18 chromatographic-type separation of component compounds occurs, effective 19 reduction of oil-water interfacial tension still takes place. As a corollary, variations in salt concentrations within the formation, e.g.
21 due to mixing or concentration inhomogeneities, do not affect surfactant 22 performance to the extent experienced with surfactants used previously.
23 ~s mentioned previously, the surfactant system may be further 24 optimized by choosing the specific appropriate propylene oxide and ethylene oxide combination. This simply means that a choice is made as 26 ~o the component compounds which predominate in the distribution of 27 compounds which comprises the surfactant. Thus, it has been found that, 28 at a given temperature and for a given crude oil, an optimum combination 7 ~

1 of propoxy and ethoxy groups in a surfactant exists for various salt concen-2 trations. A family of curves defining this relationship can be derived 3 (for example, the optimal salinity map of the drawing). This family of 4 curves may be used in determining the best surfactants (i.e. set {(m, n)}) for a given salt concentration for a particular formation and crude oil.
6 Whether a sulfate, sulfonate, phosphate, or carboxylate type 7 surfactant should be used and the relative hydrophilic-lipophilic balance 8 which will be most effective and efficient in recovering oil will depend to 9 a large extent on the physical and chemical characteristics of a particular formation. As mentioned previously, sulfates and sulfonate-type surfactants 11 have been found to be effective in general, and are therefore preferred.
12 Moreover, use of a C10 16 branched alcohol as the alkoxylation precursor is13 also preferred, since a good hydrophilic-lipophilic balance has been found 14 to be achieved thereby.
The surface-active agents of this invention can be used in any 16 flooding process where a surfactant is introduced into a formation for the 17 purpose of recovering crude oil. Accordingly, the process of the present 18 invention finds application in aqueous surfactant solutions. Still further, 19 the surface active agents may be used in liquid hydrocarbon solution in those recovery techniques in which an oil solvent is employed to provide a 21 miscible displacement of the crude oil within the formation. The surfactants 22 also have special applicability in "microemulsio~s."
23 The expression "microemulsion" as used herein is defined as a 24 stable, transparent or translucent micellar solution of oil, wat~r, and oneor more surfactants. The solution may optionally contain co-sur factants 26 and/or cosolvents. These microemulsions may be water-ex~ernal, oil-external, ~Lt~ 77~

1 or may fall into that class of micellar structures in which there is appar-2 ently no identifiable external phase. The microemulsions may be single-3 phase solutions which can take up additional quantities of oil or water 4 without phase separation. The microemulsions may be immiscible with oil, water, or both.
6 An effective microemulsion for an oil recovery process must 7 sufficiently displace oil, and in turn the microemulsion must be effec-8 tively displaced by any water which drives it through the formation. To 9 satisfy these criteria both the microemulsion-oil and microemulsion-water interfacial tensions must be low when phase separation occurs.
11 The microemulsion for use in a specific applicatio~ is designed 12 by first ascertaining information concerning the oil-bearing formation from 13 which oil is to be recovered. The oil from the formation is analyzed to 14 determine its physical and chemical characteristics. Similarly, water from ~5 the formation is analyzed to determine the quantity and type of ionic 16 substances present. The formation temperature is also determined by conven-17 tional means.
18 Microemulsions are then for~ulated on the basis of the infonma-19 tion obtained from the subterranean formation. It is preferred that the oil be one which has physical and chemical characteristics approximating 21 the characteristics of the crude oil of the subterranean formation. The 22 aqueous medium employed in the formation of the microemulsion of the 23 present invention can be pure water, but is preferably a brine. The best 24 salt concentrations for any particular microemulsion system will depend, among other criteria, on the salt concentrations of the formation brine.
26 Therefore, most preferably formation brine, or an aqueous medium haYing 27 similar physical and chemical characteristics thereto, is ernployed.

-~3 ?~

I The third essential component of the microemulsion of the 2 present inven~ion is a surfactant having a formula as characterized by 3 this invention. A suitable surfactant is i-tridecyl ether (PO)m (EO)n 4 sulfate, wherein the values of m and n preferably are selected using guidance from the drawing for a given salt concentration to further 6 optimize the surface-active properties of the surfactant for a particular 7 reservoir. Thus, for use in displacing at 74F a crude oil having 8 properties similar to diesel fuel in a formation containing the approxi-9 mate equivalent of 110% TSB, a particularly suitable surfactant is i-C13H270(P0)4(E0)3S03 Na . It must be emphasized at this point that ll similar information cannot be obtained from the drawing for a different 12 temperature, crude or qualitatively different brine. Rather, the drawing 13 must be appropriately reconstructed.
14 In preparing the microemulsions of the present invention, the proportions of oil, water, and surfactant are not particularly critical 16 as long as the same are sufficient to provide a microemulsion. Accord-17 ingly, the amount of water and the amount of oil can vary within wide 18 limits. It is noted, however, that it has been discovered that the l9 method of the present invention is applicable to the use of both water-external microemulsions and oil-external microemulsions as well as those 21 micellar structures in which no particular external phase is discernible.
22 Therefore, in the microemulsions of this invention, it is sufficient 23 that the surfactant be employed in an amount effective to produce the 24 desired microemulsion. For most purposes, the surfactant is employed in an amount from about 1% to about 20% based upon the volume of the micro-26 emulsion, and preferably between about 2% and about 12%. Usually, the 27 lowest concentration possible is utilized so that a correspondingly 28 large bank size can be used to compensate for deleterious effects of 29 reservoir heterogeneity, and the upper limits are based upon economic considerations.

-2~-'7~-3 1 As indicated previously, the microemulsion can contsin one or 2 more co-surfactants and/or co-solvents. The co-surfactants or co-3 solvents can be employed, for example, to adjust the viscosity of the 4 microemulsion. Generally, the co-solvent is employed in an amount of S from about 0.1% to about 10% based upon the volume of the microemulsion.
6 Many surface-active materials having a lipophilic portion can be effect~
7 ively utilized as a co-surfactant or co-solvent in the environment of 8 the present invention. The co-surfactants or co-solvent which have been 9 found to be particularly effective include, but are not limited to, alcohols, ethoxylated alcohols, sulfated ethoxylated alcohols, sulfonated 11 ethoxylated alcohols, ethoxylated phenols, sulfated ethoxylated phenols, 12 and synthetic sulfonates. The alcohols which are used as co-sol~ents 13 are generally C3 20 aliphatic alcohols including, for example, isopropanol, 14 isobutanol, t-butanol, and various amyl alcohols, 1 and 2-hexanols, 1 and 2-octanols and dodecanol.
16 Although co-surfactan~s and co-solvents can sometimes be used 17 to advantage, it is a noteworthy and significant feature of the surfac-18 tants of this invention that such co-surfactants or co-solvents are 19 often not required. This is thought to be a conse~uence of the complex nature of these surfactants inasmuch as many values of m and n are 21 represented. Hence, a whole range of co-surfactants and co-solvents is 22 already "built~in."
23 In addition to the above, the fluids containing the surfactants 24 of the present invention (e.g. microemulsions, flooding water, brines, etc.) can optionally include a thickener for mobility control. Typical 2~ thickeners include water-soluble polymers including polysaccharides, 27 such as those sold under the ~rr~etone "~elzan XC" by Kelco Corporation;

~ t~ r~
28 another typical example is Biopolymer 1~35~sold by Pfi~er, Inc. Other 29 thickeners include high molecular weight polyacrylamides and acrylamide 77C ~3 copolymers, more specifically, for example, partially hydrolized polyacryl-amiAes such as those sol.d under the trademark "Pusher" by Dow Chemical Company. Another specific example of a suitable thickener is the improved hiopolymer of U.S. Patellt No. 4,182,860. The thickeners are employed in the microemulsion in an amount sufficient to create a favorable mobility ratio between the microemulsion and the fluids being displ.aced by the microemulsion.
~ urther, it has been found that an additional and important advant-age of surfactants of the present invention is that they exhibit good compat-ibility characteristics with such mobility control agerlts. Many known sur-factants cannot be utilized with such agents hecause of various incompati-bility problems, such as coprecipitation or loss of surface activity.
Microemulsions for injection into a formation and which use sur-factants of the present invention are further illustrated by the following Table II, wherein all components are reported as vollLme percents based on the total microemulsion volume, except that inorganic salts are reported in y/l .

2 Table II - Typical Formulations of Microemulsions for Injection 3 Percent Ranges 4 Component General Preferred Oil 0.25-g0% 0.25-15%
6 Water 1-9S% 80-95%
7 Surfactant 1-20% 2-12%
8 Inorganic Salts 0-250 g/l equivalent to 9 reservoir brine Co-surfactant 0-15% 0%
11 Thickener 0- 2% 0%

12 The surfactant used in formulations prepared according to Table II
13 are generally characterized by compound "a" and the particular compound 14 used will be governed by the principles discussed previously and, more specifically, by the concentration of inorganic salts in the formation 16 brine. In particular, the average values of m and n will be determined by 17 the inorganic salt content. Most preferably, the concentration of inorganic 18 salts will be substantially equivalent to that of the formation brine of 19 the reservoir being flooded.
In practice, the microemulsion is first injected into the subter-21 ranean formation in the form of a slug fsllowed by the injection of thickened 22 water and thereafter unthickened water. The slug of microemulsion is 23 injected into the subterranean formation in a qnantity selected to be large 24 enough to effectively displace the crude oil in the formation to one or more production wells. Those skilled in the ar~ can determine the volume 26 to be injected. The thickened water which is injected after the slug of 27 microemulsion can be any conventional thickened wa~er used as a driving 28 fluid in microemulsion flooding processes. Following thP injection of 29 thickened water, unthickened water is injected as a flooding medium. Any of these fluids can contain the surfactants of the present invention.

r~ D~

1 The thickened and unthickened water act as driving fluids to 2 drive the microemulsion slug through the subterranean formation and the 3 microemulsion slug displaces crude oil trapped therein. The displaced oil 4 is driven to the production means and then to the surface of the earth.

Comparative Tests 6 The invention is further illustrated by the following tests which 7 were performed to determine ~he surface active properties of surfactants of 8 the present invention. Similar tests were performed using surfactants 9 wherein the precursor has been (a) e~hoxylated-only; (b) propoxylated-only;
and (c) ethoxylated and then propoxylated.
11 The experimental conditions and results are presented in Table 12 III. In comparing results, three types of values will be focused upon: a) 13 solubilization parameters for oil in microemulsion~ volume ratio of oil to 14 surfactant in microemulsion phase (VO/V ); b) optimal salinities for phase behavior, percent TSB (C~); and c) final oil saturations, dimensionless 16 (S f). The conditions and microemulsion compositions for core flooding 17 experiments to obtain S f values are reported in Table III and Example 4 18 following. The procedure of Example 3 following was used to determine 19 solubilization parameters and opti~al salinities.
The solubilization parameter, V /V , has been found to provide a 21 good indication of surface activity. The higher the solubili~ation parameter, 22 the lower the interfacial tension will be hetweetl the microemulsion and the 23 oil after phase separation occurs. Therefore, it is desirable to have high 24 V /V ~alues.
o s The optimal salinity, C~, for a given surfactant approximately ~6 equals that concentration of inorganic salts in the ~icroeml~lsion at which -2~-r~ ~J ~ ~

1 a ]ow interfacial tension exists for both the microemulsion-oil interface 2 and the microemulsion-water interface. Thus, it represents an optimum 3 balance of acceptably low interfacial tensions. It will be noted that the 4 data from Table III for C~ versus m and n do not always exactly fall on the appropriate curve from the family of iso-C~ curves shown in the drawing.
6 The reason for this is that uncertainty in the data requires the use of an 7 averaging procedure in determining the best location for each curve.
8 Balanced, low interfacial tensions are important since this means 9 that, on the one hand, the microemulsion can effectively displace oil from the formation and, on the other hand, the microemulsion itself can be 11 displaced by driving fluid (e.g. thickened water). In the drawing, C~ is 12 expressed as percent Tar Springs Brine (TSB), previously defined in Table 13 I. This brine contained approximately 104,000 ppm total dissolved solids 14 ~TDS), of which Na+ and Cl ions accounted for about lO0,000 ppm. The remaining 4,000 ppm of TDS were accounted for by Ca++ and Mg++ ions and 16 other trace substances.
17 The final oil saturation, S f~ indicates the ultimate amount of 18 residual oil in the formation after the flooding process has been completed.

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1 Example 3 following will illustrate the preparation of various 2 microemulsion systems for determining C~ and solubilization parameters, and 3 Example 4 will illustrate the core flooding tests. It should be understood 4 that the invention is in no way limited to these examples. In the examples, brine, surfactant and oil concentrations are presented as percent by volume.

6 Example 3 7 Microemulsion compositions for determining C~ and V0/Vs reported 8 in Table III were prepared by mixing equal volumes of water containing 9 varying proportions of dissolved solids (related to the TDS of the formation brine from the field) and a diesel oil. The oil used approximated the 11 physical and chemical characteristics of a particular formation crude oil 12 (Loudon crude).
13 The surfactants added to the solutions all utilized an alkoxylated 14 i-tridecyl alcohol substrate. The alkoxylsted i-tridecyl alcohols had then been sulfated and neutralized as described previously. The surfactant 16 concentration in all cases was about 1% by volume. These mixtures were 17 agitated to permit thorough mixing and then permitted to stand for phase 18 separation. The volume of oil and volume of water taken up by each micro-19 emulsion was measured, and the volumetric ratio of oil to surfactant (V0/Vs) was calculated. The volumetric ratio of water to surfactant (Vw/V6) was 21 also determined.
22 The solubilization parameters (V0/Vs and Vw/Vs) for each micro-23 emulsion system were determined by varying the salinity of the aqueous 24 phase in the microemulsion. By plotting the resulting parameter values versus salinity, their intersection determines the optimal salinity (C~).

26 A more complete description of the testing methods used herein to 27 obtain the various experimental values is given in U.S. Patent No. 3,981,361 -3]-~1~377~0 1 (issued to R. N. Healy on September 21, 1976). Further background informa-2 tion may be found in "A Laboratory Study of Microemulsion Flooding," R. N.
3 Healy, R. L. Reed, and C. W. Carpenter, Jr., Society of Petroleum Engineers 4 Journal, Feb. 1975, pp. 87-103; also in "Multiphase Microemulsion Systems", R. N. Healy, R. L. Reed, and D. G. Stenmark, Society of Petroleum Engineers 6 Journal, June 1976, pp. 147-160.
7 Various microemulsions were then prepared having compositions as 8 defined in Table III and were subjected to core displacement tests using 9 slugs of each microemulsion driven by thickened water having a salinity of about 70% TSB. In general, each microemulsion tested had been prepared at 11 or near its optimal salinity for the particular surfactant and oil used in 12 the microemulsion. The only microemulsions of Table III which were not 13 formulated under optimal salinity conditions for displacement tests were 14 for materials A, C and I.
Example 4 is presented to illustrate the procedure followed in 16 core displacement tests of the various microemulsions listed in Table III.

18 The selected microemulsion was injected into a Berea sandstone 19 core to determine the efficiency of an oil recovery process. The core used in these tests was a section of Berea sandstone having a cross-section of 21 1 inch by 1 inch and a length of 24 inches. Each test core had a permeabil-22 ity to brine of approximately 400 millidarcies and was mounted in epoxy 23 with tap-fittings at each end for injection and production of fluids.
24 Prior to conducting the displacement test, the cores were flooded 2~ with oil and brine to approximate the oil and water saturations that would 26 exist in an oil reservoir which had been flooded to the point that no 115~77~

1 further oil could be produced. In this flooding operstion, the core was 2 first saturated with a brine solution having 104,000 ppm TDS of the composi-3 tion described previously (TSB). The core was then flooded with a 32-34 4 API gravity oil from the Loudon Field in Illinois, until no further brine could be produced. The core was then once again flooded with brine to 6 remove all of the oil which could be recovered by a conventional water-7 flooding process. At this point, the quantities of oil and water remaining 8 in the cores approximated those in a reservoir which had been water-flooded 9 to a residual oil saturation. The residual oil in this core was approxi-mately 35% of the pore volume of the core; the remaining 65% was saturated 11 with brine. Precise values are listed in Table III.
12 After the core had been water-flooded to a residual oil æaturation, 13 the following displacement test was conducted in the core. A microemulsion 14 solution having a composition listed in Table III was injected into the water-flooded core. Injection of this microemulsion solution was continued 16 until approximately 0.2 pore volumes of fluid had been injected. The 17 microemulsion solution was followed by a brine solution containing about 18 750 ppm of a biopolymer thickening agent in 70% TSB until oil production 19 ceased. The viscosity of this brine solution was about 12 centipoises.
These fluids were injected at an average frontal velocity of approximately 21 one foot per day. The final oil saturation was obtained by measuring the 22 incremental oil recovery.
23 Comparison of the results reported in Table III illustrates the 24 advantages of surfactants of the present invention. In comparing results, it should be mentioned that surfactants K-M are single component surfactants 26 (i.e. surfactants which have been purified extensively to yield a single 27 species). The remaining surfactant materials are distributions of compounds, 28 as discussed previously.

`77~

1 Comparison of VO/VS Values 2 From Table III it can be seen that, where available, the solubili-3 zation parameters (VO/V ) for the propoxylated ethoxylated material (sur-4 factants A-E 0-Q) are uniformly high. Values of about 15 are considered to be very good. By contrast, the values for surfactants which contain only 6 propoxy groups (surfactants F-H) do not demonstrate the same high V tVs.
7 In fact, surfactant F does not even produce a fluid microemulsion, but 8 instead gives a gel. This indicates that the propoxylated ethoxylated 9 material would be expected to give much lower overall interfacial tensions than the propoxylated-only material.
11 An even more substantial difference is seen in comparing the 12 propoxylated ethoxylated material with the reverse-ordered ethoxylated 13 propoxylated material. (See Table III, surfactants R-T.) The solubilization 14 parameters for the reverse-ordered materials are extremely low and indicate that unacceptably high interfacial tensions would result.
16 While the V /V value available for the ethoxylated-only material 17 (surfactants I-N~ indicates that the m=0, n=2 species might be acceptable, 18 comparison of optimal salinities (C~) indicates that the use of these 19 surfactants in field flooding would present significant problems with chromatographic-type separation in the formation.

21 Comparison of C~ Values 22 The C~ values for the ethoxylated-only materials (surfactants I-23 N) demonstrate the potential problems for field operations. At one end of 24 the scale, surfactant K (with only one ethoxy group~ gives a gel. Each time an ethoxy group is added (surfactants I and J, and L-N), a large jump 26 in optimal salinity results. Thus, a change of one ethoxy group in moving ~ ~3~7 ~J C ~

1 from surfactant I to surfactant J, causes a jump in optimal salinity of 2 about 45%. [By contrast, a change of two alkoxy groups between surfactants 3 A and B (one propoxy and one ethoxy group) causes a drop in optimal salinity4 of only 20~.1 The large jumps in C~ values for ethoxylated-only materials S foreshadow severe problems in the formation as the various components 6 separate and are subjected to formation brine having concentrations of 7 inorganic salts which are not optimum for the separated components. Thus, 8 one discrete component may precipitate when existing separately in the 9 formation brine, while another may form a gel under these conditions and a third may exhibit totally inadequate surface active properties.
11 Moreover, while the ethoxylated-only material C~ values range 12 from no optimal value (gel) to greater than 220% over a relatively narrow 13 range of ethoxy groups, the propoxylated-ethoxylated material C~ values 14 range from 96% to 135% TDS over a very broad range of m and n values. Thisfurther illustrates why many of the components of surfactants of the present 16 invention may be separated during flooding, and yet still retain good 17 surface activity in reservoir brine.
18 An advantage of surfactants of the subject invention is further 19 illustrated by reference to the drawing where all currently available data (for similar conditions) on optimal salinities of surfactants which are 21 ethoxylated-only (m=0), propoxylated-only (n=0), and propoxylated and then 22 ethoxylated (all of the form i-C13H270(PO~m(EO)nS03Na for m, n=0, 1, 2, 3,23 4, 5, 6) have been plotted. The best smooth curves have been drawn through24 the data so as to make the iso-optimal salinity contours evident.
Suppose it is required to design a surfactant for use in prepara-26 tion of a microemulsion to displace crude oil from a reservoir at 74F
27 containing a crude having properties similar to the diesel oil used in `77~

1 obtaining the data of the drawing. Suppose further the resident brine salinit~
2 equivalent to 140% TSB. Then, as discussed previously, all surfactants 3 lying on or close to the contour labelled 1.4 will be suitable candidates 4 for such a microemulsion. Inspection of the drawing shows that there are no ethoxylated-only (m=0) surfactants that fall anywhere near the design 6 contour.
7 Table IV summarizes possible surfactants and illustrates the 8 extent that the various C~ values deviate from that value desired (140%
9 TSB). Listed therein are those ethoxylated-only surfactants that lie lC nearest the contour labelled 1.4 and also the values of a parameter ~.
11 This parameter is the absolute value of the difference between the desired 12 optimal salinity and the actual optimal salinity for the surfactant in 13 question, expressed in % TSB. For ethoxylated-only surfactants, the smallest 14 deviation shown is 30% TSB. Hence, Table IV illustrates that there are no ethoxylated-only surfactants clearly suitable for use in the above described 16 reservoir.
17 Similarly, Table IV illustrates that there are no propoxylated-18 only (n=0) surfactants clearly suitable for use in this reservoir.
19 However, further inspection of Table IV shows that for the surfac-tants of this invention, which have been propoxylated first and then ethoxy-21 lated, there are a large number of candidates for preparation of a micro-22 emulsion suitable to recover oil from the above described reservoir.
23 In fact, suppose the surfactant i-C13H270(P0)3(E0)4S03Na is 24 selected. Then, among the many pure-component compounds that make up this complex mixture having an average value of m equal to 3 and an average 26 value of n equal to 4, Table IV shows that there are at least 9 surfactants 27 that have optimal salinities deviating from the design salinity of 140% TSB

-~7~

1 by no more than 10% TSB. There are at least 6 that deviate by no more than 2 2% TSB, and there is one that has exactly the correct value of optimal 3 salinity so that its corresponding deviation is zero% TSB.
4 In addition to those compounds listed in Table IV under the heading "Propoxylated and Ethoxylated," there will be other suitable com-6 pounds having larger values of m and n than those shown and which exhibit 7 good optimal salinities. In fact, extrapolation of the trend established 8 in the drawing suggests that there are at least 14 different pure-component 9 compounds of the structure taught by this invention that have optimal salinities that deviate no more than the equivalent of 11% TSB from the 11 design salinity and that occur in significant mole-fractions within the 12 mixture. This multiplicity of surfactants within the mixture that have 13 high interfacial activity is further evidence of the advantages that accrue 14 to a surfactant of the general formula of compound (a) in that there is great flexibility to meet design criteria by suitable adjustment of the 16 average values of m and of n.

17 Table IV - Comparison of C~ Deviations 18 Propoxylated and 19 Ethoxylated-Only Propoxylated-Only Ethoxylated _ _ ~ _ n ~ m n a 21 0 1 Gel 1 0 Gel 1 1 2 ~ = ¦140 - C~l where C~ is the optimal salinity in %
31 TSB for the surfactant having the stated values of m and 32 n in the form~la i~C13H27(PO)m(EO)n 3 ~ r-~

1 Comparison of Sof Yalues 2 Surfactants A-E and 0-Q all have the general formula of compound 3 (a) and were manufactured in accordance with the teachings of this invention 4 wherein propoxylation is followed by ethoxylation. Very ~ood oil recoveries were obtained for surfactants B, D, and Q. Moderately good oil recovery 6 was obtained for surfactant E. In all of these floods, the core and the 7 microemulsion contained brine near the optimal salinity for each system.
8 Note further that Q is a 50/50 mixture of 0 and P, thus demonstrating that 9 surfactants of type (a) can often be mixed together and yield good oil recovery, provided the optimal salinity of the mixture has the appropriate 11 value. Although the solubilization parameters for 0, P and Q have not been 12 measured, they should be high. The remaining two floods conducted using 13 propoxylated-ethoxylated surfactants, namely A and C, did not give good oil 14 recovery because they were not run at optimal conditions. Inspection of Table III shows C~(A) = 120% TSB and C~(C) = 135% TSB; however, the resident 16 brine in the core and the brine used to prepare the microemulsions injected 17 was 100% TSB iu both cases. Hence, good oil recovery would not be expected.
18 In the case of surfactant R, the order of oxyalkalation was 19 reversed so that an average of two moles of ethylene oxide were added to the i-trideyl alcohol first and then an average of 4.2 moles of propylene 21 oxide were added to the ethoxylated material. Even though this flood was 22 carried out at optimum conditions, the oil recovery was poor, as would be 23 anticipated on the basis of the low value of 3.5 for the solubilization 24 parameter. This result demonstrates the importance of propoxylation followed by ethoxylation.
26 Surfartants F, G and H are of the propoxylated-only type (n=Q).
27 ~o flood could be run for surfactant F since it formed gels. Surfactant G

~1~77~

1 did not give good oil recovery and surfactsnt H gave only moderately good 2 recovery. Floods using surfactants G and H were, nonetheless, run at 3 conditions close to optimal. This result illustrates that propoxylated-4 only surfactants often give unsatisfactory oil recovery even when floods are run under close to optimal conditions. Further, floods corresponding 6 to F-H are generally inferior to those corresponding to surfactants of this 7 invention such as, for example, B, D and Q.
8 Surfactants I-N are all of the ethoxylated-only type (m=O), but a 9 flood was run only in the case of surfactant I. There, the oil recovery was moderately good, but clearly inferior to B, D, E and Q. The resultfi of 11 these tests demonstrate that use of the surfactants of the present invention 12 has significant advantages over the use of those of the prior art. In 13 addition, the use of the propoxylated ethoxylated surfactants permits 14 greater latitude in formulating the microemulsions.
The principle of the invention and the best mode contemplated for 16 applying that principle have been described. It is to be under~tood that 17 the foregoing is illustrative only and that other means and techniques can 18 be employed without departing from the true scope of the invention defined 19 in the following claims.

Claims (31)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method for recovering oil from an oil-bearing formation which comprises injecting into said formation a fluid containing an effective amount of surface-active agent having the general formula:
R1O (C3H6O)m (C2H4O)n YX
wherein R1 is a linear or branched alkyl radical, an alkenyl radical, or an alkyl or alkenyl substituted benzene radical, the non-aromatic portion of the radical contain-ing from 6 to 24 carbon atoms;
m has an average value of from about 1 to about 10;
n has an average value of from about 1 to about 10;
Y is a hydrophilic group; and X is a cation; and driving said fluid through said formation to displace oil from said formation and recovering the displaced oil.

2. The method of claim 1 wherein Y is chosen from the group consisting of the sulfate, sulfonate, phosphate and carboxylate radicals.

3. The method of claim 1 wherein Y is a sulfonate having the general formula wherein R2 is an alkyl, cycloalkyl, alkenyl, alkylaryl, or aryl radical containing up to 8 carbon atoms; and R3 is a hydrogen atom, a hydroxyl radical, or an aliphatic radical containing from 1-8 carbon atoms.

4. The method of claim 1 wherein R1 is a branched alkyl substi-tuent containing from 10 to 16 carbon atoms.

5. The method of claim 1 wherein m has an average value of about 2 to about 6 and n has an average value of about 2 to about 6.

6. The method of claim 1 wherein said fluid is an aqueous fluid containing inorganic salts.

7. The method of claim 6 wherein said aqueous fluid is brine from said formation.

8. The method of claim 6 wherein the values of m and n are varied with respect to each other in proportion to the concentration of said inorganic salts in said aqueous fluid so as to optimize the surface activity of said surface-active agent.

9. The method of claim 1 wherein said fluid is a microemulsion.

10. The method of claim 9 wherein said microemulsion is driven through said formation by a slug of thickened water.

11. The method of claim 9 wherein said microemulsion further includes a co-surfactant.

12. The method of claim 9 wherein said microemulsion further includes a co-solvent.

13. The method of claim 9 wherein said microemulsion includes a thickener.

14. The method of claim 9 wherein said microemulsion contains inorganic salts.

15. The method of claim 14 wherein the values of m and n are varied according to a set of curves defined at varying concentrations of said inorganic salts, said curves being generated by measuring optimum salinities as a function of m and n at each of said varying concentrations.

16. The method of claim 9 wherein the precise values of m and n are selected from a plot obtained by:
(a) preparing a plurality of surface-active agents characterized by said general formula and having different average values of m and n;
(b) preparing a microemulsion sample which includes one of said plurality of surface-active agents, water, oil, and inorganic salts, the concentration of said salts being such that a balanced minimum microemulsion-oil and microemulsion-water interfacial tension is obtained;
(c) repeating step (b) for each of said plurality of surface-active agents;
(d) plotting those values of m versus n which give minimum microemulsion-oil and microemulsion-water interfacial tensions at one concentra-tion of said salts, thereby generating a curve;
(e) repeating step (d) for other concentrations of said salts to generate a family of curves, each curve defining values of m and n giving balanced minimum interfacial tensions at different concentrations of said salts.

17. The method of claim 1 wherein said surface-active agent is an i-tridecyl ether (C3H6O)m (C2H4O)n sulfate, the value of m ranging from about 2 to about 6 and the value of n ranging from about 2 to about 6.

18. The method of claim 17 wherein said formation contains high concentrations of inorganic salts.

19. A method of enhanced oil recovery which comprises injecting a microemulsion containing an effective amount of surfactant having the general formula:
R1O (C3H6O)m (C2H4O)n YX
wherein R1 is a C10 16 branched chain alkyl radical having at least two branching groups;
m has an average value of between about 2 and about 6;
n has an average value of between about 2 and about 6;
Y is the sulfate or sulfonate group; and X is a cation; and driving said microemulsion through said formation to displace oil to a producing well.

20. The method of claim 19 wherein R1 is an i-tridecyl radical.

21. The method of claim 19 wherein said surfactant is present in an amount ranging from about 1 to about 20 volume percent based or. the volume of said microemulsion.

22. The method of claim 21 wherein the range is from about 2 to about 12 volume percent.

23. A surfactant useful in enhanced oil recovery operations having the general formula:

R1O (C3H6O)m (C2H4O)n YX
wherein R1 is a linear or branched alkyl radical, an alkenyl radical, or an alkyl or alkenyl substituted benzene radical, the non-aromatic portion of the radical contain-ing from 6 to 24 carbon atoms;
m has an average value of from about 1 to about 10;
n has an average value of from about 1 to about 10;
Y is a sulfate, sulfonate, phosphate or carboxylate group; and X is a cation.

24. The surfactant of claim 23 wherein Y is a sulfonate having the general formula wherein R2 is an alkyl, cycloalkyl, alkenyl, alkylaryl or aryl radical containing up to 8 carbon atoms; and R3 is a hydrogen atom, a hydroxyl radical, or an aliphatic radical containing from 1-8 carbon atoms.

25. The surfactant of claim 23 wherein R1 is a C10-16 branched chain alkyl radical having at least two branching groups.

26. The surfactants of claim 25 wherein m has an average value of from about 2 to about 6 and n has an average value of from about 2 to about 6.

27. A formulation suitable for injection into a hydrocarbon-bearing formation for enhanced recovery operations which comprises:
(a) 0.25-90% by volume of oil;
(b) 1-95% by volume water;
(c) 1-20% by volume of a surfactant having the general formula:

R1O(C3H6O)m(C2H4O)nYX
wherein R1 is a linear or branched alkyl radical, an alkenyl radical, or an alkyl or alkenyl substituted benzene radical, the non-aromatic portion of the radical contain-ing from 6 to 24 carbon atoms;
m has an average value of from about 1 to about 10;
n has an average value of from about 1 to about 10;
Y is a sulfate, sulfonate, phosphate or carboxylate group; and X is a cation;
(d) 0-250 g/l inorganic salts;
(e) 0-15% by volume of a co-surfactant; and (f) 0-2% by volume of a thickener.

28. The formulation of claim 27 wherein Y is a sulfonate having the general formula wherein R2 is an alkyl, cycloalkyl, alkenyl, alkylaryl or aryl radical containing up to 8 carbon atoms; and R3 is a hydrogen atom, a hydroxyl radical, or an aliphatic radical containing from 1-8 carbon atoms.

29. The formulation of claim 27 wherein R1 is a C10-16 branched chain alkyl radical having at least two branching groups.

30. The formulation of claim 29 wherein m has an average value of from about 2 to about 6 and n has an average value of from about 2 to about 6.

31. The formulation of claim 27 wherein said formation contains a brine having a known composition of inorganic salts and wherein the amount of components (a) - (d) are respectively:
(a) 0.25-1%
(b) 80-95%
(c) 2%-12%
(d) substantially the same as that of said brine and components (e) and (f) are omitted.